Electromechanical reverberation device



Oct. 25, 1966 w. SCHAFFT 3,281,724

ELECTROMECHANICAL REVERBERATION DEVICE Filed Nov. 19, 1963 2 Sheets-Sheet 1 21 Y 20 63 INVENTOR 3 2 BY 2 4M H. W. SCHAFFT ELECTROMECHANICAL REVERBERATION DEVICE Oct. 25, 1966 2 Sheets-Sheet :2

Filed Nov. 19, 1965 I INYENTOR.

REVERE AM P SOURCE United States Patent 3,281,724 ELECTROMECHANICAL REVERBERATION DEVICE Hugo W. Schalr't, Des Plaines, Ill., 'assignor to Motorola Inc, Franklin Park, Ill., a corporation of Illinois Filed Nov. 19, 1963, Ser. No. 324,638 '7 Claims. (Cl. 333-30) This invention relates to devices for producing delayed decaying electrical signals, and more particularly to a reverberation device which may be used in connection with sound reproduction apparatus fior electrically simulating the acoustic conditions of a large room.

Certain musical performances, such as a symphony orchestra concert, are generally given in a large hall wherein the music will reach the ear of the listener via a direct path and via several other reflecting paths that are caused by sound waves bouncing oil the walls, ceiling, and other reflecting surfaces of the hall. Numerous reflections will therefore reach the ear of the listener in a we cessive order, the amplitude thereof decreasing exponentially. Because commercially available recordings or broadcasts often contain no reverberation, when such sound is reproduced in living room or automobile of a listener, the smallness of the enclosure will generally preclude the occurrence of any detectable reverberation.

It is therefore often desirable, in sound reproduction systems which are used in rooms that are smaller than the rooms in which the sound being reproduced is normally heard, to utilize a reverberation device. Such a device will produce delayed, decaying electrical signals to simulate the acoustical characteristics of a arge room, auditor-ium, or concert hall with a sound source located therein at some distance from a listener. The concert hall effect thus attained achieves a subjective enlargement of the room or enclosure in which the listener is located.

Numerous w-ays have been proposed to produce delayed decaying electrical signals. One of those ways is to change the electrical signals to mechanical vibrations to be transmitted along a vibration transmitting path which delays the transmission and produces echos or reflections, which are then TC-OOIIVCTlGd into electrical signals for the desired reverberation effect. Devices providing sufficient delay within a reasonable range of frequencies, and providing sufiicient reflections such that their separate arrival times are indistinguishable by ear, have been relatively complex and expensive. Furthermore, such devices are often excessively large and are sometimes very sensitive to vibrations from outside sources. This latter defect can be especially annoying in automobiles.

Accordingly, it is an object of this invention to provide an improved reverberation device closely simulating the acoustical characteristics of the concert hall.

Another object of the invention is to provide a compact reverberation device which is low in cost and simple of construction.

Still another object of the invention is to provide a device for producing delayed decaying electrical signals which produces enough reflections to render their separate arrival times indistinguishable by ear, and which unit is relatively insensitive to outside vibration.

A further object of the invention is to provide an improved high efiiciency transducer device tor a reverberation unit, and a mounting for such device wherein all but torsional vibrations are damped.

A feature of the invention is the provision, in a device for producing delayed decaying electrical signals, of a pair of vibration transmitting members, each having one end connected to a transducer device and the other end suspended. A link couples vibrations between the two members at a point intermediate their ends to provide a primary vibrational path between the transducers, and a plurality of secondary paths of different lengths for reflected vibrations.

Another feature of the invention is the provision, in a device as described, of structure for suspending the ends of the vibration transmitting members to reflect vibrations therein in different phase relationships.

Still another feature of the invention is the provision of input and output transducers connected to an end of respective helical springs, with the opposite end of one spring suspended to reflect torsional vibrations in phase and with the opposite end of the other spring suspended to reflect torsional vibrations out of phase. One spring constitutes la one-quarter wavelength resonator while the other consitute a ione half wavelength resonator. 'Ihe springs are oppositely wound and a link intermediate the middle and the reflective ends thereof couples torsional vibrations between the springs.

Another feature of the invention is the provision of a transducer device including a pair of adjacent levers and an energy converting element for displacing a first end of each of the levers according to an electrical signal, and for producing an electrical signal according to displacement of those ends. The elevers are disposed so that the second ends move in opposite directions to produce torsional movement upon displacement of the first ends, and such that displacement of the second ends in opposite directions due to torsional movement causes displacement of the first ends.

In the drawings:

FIG. 1 is a top plan view of a device constructed in accordance with the invention;

FIG. 2 is a perspective View of a portion at the upper end of the device shown in FIG. 1;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 1;

FIG. 4 is a perspective view of the transducer device of the invention;

FIG. 4a is a perspective view of an alternative construction for the transducer device of FIG. 4;

FIG. 5 is a schematic showing of the transducer device of FIG. 4;

FIG. 6 is a perspective view of the transducers and connection for the right hand spring shown in FIG. 1;

FIG. 7 is a sectional view taken along the line 77 of FIG. 1;

FIG. 8 is a perspective view of the mounting block for the transducer shown in FIG. 4; and

FIG. 9 is a block diagram of a circuit in which a device constructed in accordance with the invention may be incorporated.

A reverberation unit constructed in accordance with the invention comprises two identical torsional transducers, two helical springs of equal length, a coupling link between the ends of the springs, and a housing for carrying these elements. One of the transducers operates as a driving element while the other operates as a receiving element. One side of the first spring is connected to the driver transducer while its opposite end is suspended in such a way as to rotate freely. One end of the second spring is connected to the receiver transducer while its other end is connected rigidly to a mass. A coupling link intermediate the ends of the springs transmits torsional vibrations therelbetween.

The driving and receiving transducers are substantially identical. The transducers incorpoarte a ceramic bender or bimorph and a lever system which converts the bending motion of the bimorph into a force couple. When this force couple acts upon the helical spring in the reverberation unit, it" produces a torsional motion therein. The reverse is also true so that identical lever systems are used for both input and output. The lever system comprises a pair of levers which are finlcrumed intermediate their ends, and these fulcrums are fixed in relation to the center of the bimorph by a joining web. Each of the ends of the bimorph engages a respective one of the ends of each of the levers for displacing the same according to electrical signals applied to the bimorph, or in the case of the receiver transducer, for being displaced according to its drive motion to produce an electriacl signal in the bimorph. The ends of the levers opposite the bimorph move in opposite directions in response to bending of the bimorph or in response to vibration of the spring to which they are attached.

One means of attaching the levers to the springs is by a stainless steel extension of generally U-shaped configuration. The free sides of the -U-sh-aped extension are attached to respective ends of the levers, and the joining portion of the extension is connected to drive the spring. An alternative connection includes a nylon pin extendig between the two levers, the center of which is joined to the spring. The transducer is mounted in a universal mounting which is not susceptible to torsional vibrations but which is !free to move and damp out all other modes of vibration. Further damping is provided by a mounting plate upon which the elements of the reverberation device are suspended, which mounting is attached to a housing which may also be damped when mounted.

Referring now to FIG. 1, a reverberation unit constructed in accordance with the invention is shown. The unit is carried on a mounting plate 11 which in turn is attached in a housing 12 by means of rubber dampers 13 fixed to projections 14 of plate 11. A pair of transducers 16 and 1 7 are mounted on one end of the plate. The details of the transducers and their mounting will be described subsequently. Transducers 16 and 17 are nearly identical and may serve as either the input transducer or the output transducer for the reverberation unit. In FIG. 1, transducer 1 6 is the input or driving transducer whereas transducer 17 is the pick-up or receiving transducer.

A pair of helical delay springs 20 and 21 are coupled at one end to transducers 16 and 17 respectively. As will be explained subsequently, transducer 16 imparts a torsional vibration to spring 20 which is transmitted to spring 21 and received by transducer 17. Both springs 20 and 21 are identical except that they are wound in opposite directions. This is because any helix, when expanded or retracted, will produce a torsional motion, and hence springs 20 and 21 are susceptible to producing a torsional motion by a strong shock from outside of the reverberation innit. Because the springs are oppositely wound, however, the effect of such outside vibrations is virtually cancelled out. The number of turns in each spring is the same.

Referring now to FIGS. 1 and 2 the reflective means for suspending the springs at the ends opposite the transducers may be seen. Spring 20 is provided with a hook 22, and a length of string 23 (preferably of Dacron or some other similar durable material) is knotted to the hook 22 such as by a girth hitch. A lead terminal 2 4 is mounted in an upwardly extending flanged end 25 of plate 11, and string 23 is drawn through a slot 26 in terminal 24 to the position shown in FIG. 2. The two sides of notch 27 in the end of terminal 24 are then deformed over the end of string 23 to secure the string in the terminal, as shown in FIG. 1. The free end of string 23 may be cut off. The result of such a suspension is effectively equivalent to an open circuit and will reflect substantially all vibrations exactly in phase.

The hook 28 of spring 2 1 is scoured in a notch 29 in an upwardly extending flange portion 30 of plate 11. Accordingly,this end of spring 21 is rigidly secured to the mass of the plate and is effectively equivalent to a short circuit. At this end of spring 21, substantially all the vibrations will be reflected with a phase reversal that is, 180 out'of phase with the incident vibrations.

A link 33 is used to couple vibrations between springs 20 and 21 at respective points intermediate the midpoint and the suspended end of the spring. The link includes a pair of clip portions 34 and 35 which clip over the top part of a coil of springs 21 and 20 respectively. The clips 34 and '35 extend downwardly from opposite ends and opposite sides of a rigid and flexible cross bar 36. Because clips 34 and 35 are on opposite sides of cross bar 36, the coils of springs 21 and 20 to which they are clipped are at different distances from the transducers. This helps distribute the standing wave pattern for even frequency response, and also breaks up the path lengths of travel for reflected signals for even reverberation.

Placement of link 33 affects the distribution of standing wave patterns in the system. Of course the further away from the transducers the link is placed, the longer will be the delay time. The reverberation time is also affected by the link placement. It is desirable to keep the high frequency reverberation time as long as possible in relation to the low because there are so many more losses at the higher frequencies than at the lower frequencies that if this were not done an unnatural sound could result. The closer toward the suspended ends of the springs the link is placed, the more the high frequency reverberation time is emphasized. Thus the placement of the link involves a compromise of several factors including the delay time, the high and low frequency reverberation times, and the optimum distribution of standing wave patterns for even frequency response.

A reverberation unit should provide a delayed signal of decaying amplitude. The ideal delay line would consist of a vibration transmitting member such as a spring having a resonant frequency at the lowest possible number of cycles. Accordingly, the hormonics at which the particular spring is resonant will extend through the entire range of frequencies desired to be conveyed. Most of the frequencies desired to be conveyed will thereby fall at one of the harmonic peaks of the spring to be transmitted thereby at peak eificiency. The length of such a vibration transmitting member would be selected according to the delay time desired.

A difficulty with a device of the above type is the fact that the reflections of the vibrations occuring at each end of the spring will be spaced sufficiently that the arrival time of echos will be audible to the listener. In other words, a repetitious reproduction of the input signal will occur at a diminishing amplitude. To convert such a device into an acceptable reverberation unit, suficient reflective paths Wo ld have to be added to cause the reflections to occur at a rate that would make them indistinguishable. Such a modification can be extremely complex and may result in excessive size of the unit.

The reverberation device constructed in accordance with the invention utilizes a construction wherein the characteristics of a very low resonant frequency spring are achieved by utilizing two springs of a higher resonant frequency. Spring 20 is a one-quarter wavelength resonator because of the freely suspended mounting, while spring 21 is a one-half wavelength resonator because of its grounded or fixed end mounting. Harmonics will occur at odd intervals in spring 20 and even intervals in spring 21 such that when combined, the harmonic peaks fall directly in between each other and maintain that relationship. The result is an even distribution of resonant harmonic peaks at which energy may be transferred, with reinforcement or cancellation occuring due to the even distribution of peaks. Furthermore, because of the even distribution of resonant peaks, the minimum energy transfer level is also raised. By placing coupling link 33 intermediate the ends of the springs, reflecting paths are created between the link and the suspended ends to provide additional resonant combinations which further fill in the gaps between the harmonic peaks. This further raises the minimum energy transfer level at all frequencies. The various combinations of resonant frequencies and har-.

monics available thus provide an even response curve for the reverberation unit.

The foregoing device also provides sufficient reflective paths of different lengths for vibrations so that the arrival of reflected vibrations is not distinguishable by ear. When an A.C. voltage in the audio frequency range is placed on driver transducer 16, the electrical energy is converted into torsional vibration in the first helical spring 20. The mechanical impedance of the driver transducer is selected to be several times lower than the characteristic impedance of the spring 20. As the vibrational wave starts to propagate along the first spring it can travel along several paths. One path, the primary path, leads from spring 20 to coupling link 33 back along spring 22 to transducer 17. There the wave will arrive with a delay time which depends upon the delay characteristics of the spring and its length. The impedance of transducer 17 is selected to be many times lower than the characteristic impedance of coil spring 21. Accordingly, only a small fraction of the vibrational energy is converted back into electrical energy while the largest percentage is reflected. The reflected wave will again travel along several paths of which one leads along the spring 21 through coupling link 33 back along spring 20 to the transducer 16. Here again because of the mismatch condition of the impedances, most of the vibrational wave will be reflected. The vibrational wave will continue to commute between driver and receiver until its energy is fully dissipated by the inherent losses of the system. The time it takes for these reflected vibrations to decay to a given level is the reverberation time.

The construction of the reverberation unit provides a number of other possible paths for vibrational transmission. Because coupling link 36 represents a transmission irregularity, it reflects a certain amount of the vibrational energy. At the freely suspended end of spring 20, full reflection of the wave with no phase reversal takes place while at the grounded end of spring 21, full reflection with a phase reversal occurs. Because of the various places in the spring system at which reflections occur, the multitude of standing wave patterns previously discussed are set up. Together with the initial delay time and the numerous echos produced by the spring system and its various paths, the output of the reverberation unit is a delayed signal of decaying amplitude closely resembling the conditions of a concert hall.

Various relationships exist between the dimensions and properties of the system components. The fundamental resonance frequency of the spring depends upon the number of 'turns of Wire, diameter of the wire, the diameter of the coil, the density of the material of which the spring is made, and the modulus of elasticity of the spring. Of all the foregoing parameters, varying the diameter of the coil seems to be the most eflective way of altering the fundamental frequency of a spring. It will probably be desirable in the system to damp out all vibrations below 200 cycles and above 5,000 cycles. This range has been found satisfactory for a proper acoustical effect, although other circumstances may dictate that a diiference range of frequencies be used. The delay time, of course, will be determined by the length of the transmission spring between input and output transducers, and the reverberation time will be aflected by the Q of the spring. It has been found that tension on the springs does not affect any measurable parameter, however, it was noticed that a springy sound becomes apparent with increase-d tension on the spring. The diameter of the spring, and the diaineter of the wire, inversely affect the upper cut off frequency. More turns, and a 'lower cut off frequency of the spring, increases the delay. An increase in wire diameter and an increase in the modulus of elasticity increases the reverberation time. The mass of coupling link 33 inversely affects the high frequency response as does the compliance of the coupling link and of the wire of the spring.

-It will be apparent that the fore-going described con struction results in a considerably improved reverberation device. Because both springs are identical, the device is simplified. Furthermore, only one input and one output transducer is necessary and a short physical length for a given reverberation time is possible. Reinforcements and cancellations are generally avoided because of the planned placement of the harmonic peaks of resonance for a smooth response curve throughout the frequency range.

Referring now to FIG. 4, the construction of the transducer may be seen in more detail. Both transducers 16 and 17 are constructed alike and are driven by a ceramic bender or bimorph 41. The impedance of the respective benders may be selected to properly match the impedance of the driving and driven electrical circuits. The benders drive a lever system that converts the bending motion of the ceramic bender into a force couple. When this force couple acts upon opposite points on the circumference of the spring it produces a torsional motion. On the other hand, when a force couple is exerted on the ends of the lever system by torsional motion of the spring, the ceramic bender is bent to produce an electrical signal.

The lever system includes a pair of lever arms 42 and 43 which are bent longitudinally for rigidity. The arms 42, 43 are joined by an integral web 44, and the ceramic bender 41 passes through an opening in web 44 and through openings in the respective lever arms 42 and 43. Thus, the ceramic bender engages one end of each of levers 42 and 43, which levers in turn are fulcrumed at the juncture with the integral web 44. Since the ceramic bend-er extends through an opening in web 44, its center is fixed in relation to the fulcrums of respective levers 42 and 43. Epoxy cement is used to secure the bender to the levers 42 and 43 and to web 44, and also serves to insulate the bender from the metal levers and web. Wires 48 connect each side of the benders 44 to terminals on terminal board 63, so that input and output signals are conducted to .and from the respective benders.

-In FIG. 5 the lever system is shown schematically for clarity, wherein portion A represents the long portion of lever 42 and portion B the long portion of lever 43. Fortion C represents the short portion of lever 42 which is engaged by bender 41, While portion D represents the short portion of lever 43 which is engaged by bender 41. Member E represents the connecting web 44, and portions F and G represent the junctures between web 44 and levers 42 and 43 respectively. Member G represents the bender or bimorph 41. It may be seen that as member G bends to the .X position, the leverage at the ends exerts a force couple at the fulcrums represented by points F and G. This twists member E with the ends of members A and B moving in opposite directions as designated X to set up a force couple. When member G bends to the Y position, similar action causes displacement of the ends of A and B as designated Y. Conversely, it may be seen that a force couple applied to the ends of members A and B will cause opposite displacement thereof to displace points C and D in the same direction with respect to the center of member E. Accordingly, this will bend member G and produce an electrical signal therein. At the ends of levers 42 and 43 is a stainless steel U-shaped connecting members 46 (FIG. 4). The center portion of U-shaped member 46 is curved slightly to prevent twisting, and the spring 20 is rigidly cemented therein. The sides of U-shaped member 46 are respectively connected to the ends of levers 42 and 43 by cementing projections in openings in the levers. The result is a resilient extension of the levers which offers maximum stiifness to the force couple being transmitted to or from spring 20, but which offers minimum resistance or stiffness to outside vibrations for damping the same.

A damping rubber cylinder 45 is placed between the two levers 42 and 43 in order to introduce maximum damping to rflexural vibrations of the lever arms while having a negligible afie-ct on the torsional motion thereof.

FIG. 4a shows an alternative coupling between levers 42, 43 and spring 20. A nylon bearing 51 extends between the levers 42 and 43 and spring 20 is hooked about bearing 51 in a groove about the circumference thereof. In either the case of FIG. 4 or the case of FIG. 4a the compliance of the lever arms, together with the combined masses of the bearing 51 or member '46 and the hook of the spring are adjusted to resonate at a frequency near the upper cut off frequency of the entire system. This supplements the response curve of the system, which normally declines at this point, to provide a sharper response out off at the upper cut off frequency.

The transducer units 16 and 17 are mounted in universal joints to present high damping to all vibration except the torsional mode set up by the transducers themselves. This mounting is shown in FIGS. 6 through '8. The integral stamping of levers 42 and 43 and connecting web 44 is provided with a pair of mounting ears 53 and 54. These ears are cemented into notches 55 and 56, respectively, of a lead block 57, and are flexible so that the lever system may pivot in a horizontal plane with respect to the block. Lead is used for the block because its elasticity is negligible and therefore cannot transmit any vibration, isolating cross talk between the input and output transducers. Each of blocks 57 are pivotally mounted on a horizontal pin 58 by means of a damping rubber insert 59. The mass of the block together with the damping rubber insert is a low pass filter which provides the low frequency cut ofr of the transducer. The resistive component of the rubber introduces loss below the cut off frequency. Because mounting ears 53 and 54 are flexible, and because the blocks 57 are pivotal, the transducer is provided with a high loss universal hinge which allows the transducer to move freely in two planes and at the same time introduces sufficient loss to damp out low frequency flexural waves in the springs. This is very significant in damping out vibrations caused by outside sources.

The transducers are mounted on plate 11 as follows. Pin 58 is mounted in a U-shaped flange '61 formed out of a mounting plate 62 secured to support plate 11. An insulating board 63 is mounted between plate 62 and plate 11 and may carry the terminals for the electric-a1 connections to the respective ceramic bimorphs 41. The outer housing may be mounted by means of springs or damping pads, etc., to further reduce the effect of outside vibrations and to protect the system.

Referring to FIG. 9 a simplified schematic diagram of a circuit is shown in which a reverberation unit of the type described may be used. A source of audio frequency signals 71 is coupled through a preamplifier 72, through an amplifier 73, and from there to a speaker 74, where they are audibly reproduced. Some of the output of amplifier 73 is fed through a further amplifier 75 for driving a reverberation unit 76 of the type described. Output signals from the reverberation unit 76 are then fed back to the amplifier 73 so that they may be reproduced by speaker 74. By including signals of delayed decay ing amplitude in the reproduced sound, a concert hall effect is created. This results from adding to the sound reaching the listeners ears, reflected and delayed sound waves which would normally be present in a large auditorium or concert hall.

From the foregoing description it may be seen that the invention provides an improved device for producing delayed decaying electrical signals which is low in cost and simple of construction. The device produces sufficient reflections to render them indistinguishable by ear and is insensitive to outside vibration. The transducer which drives the device is an eflicient unit which produces torsional vibrations for driving the springs in the reverberation device, and the identical receiver transducer which converts torsional vibrations of the springs into electrical signals is similarly efficient.

I claim:

1. A device for producing delayed decaying electrical signals including in combination, input and output transducers for respectively producing torsional motion from electrical signals and electrical signals from torsional motion, a pair of vibration transmitting helical springs, each having an end coupled to a respective one of said transducers, means suspending the ends of said vibration transmitting helical springs opposite said transducers, and a link intermediate the ends of said springs and coupling torsional vibrations therebetween to provide a path for torsional vibrations from said input transducer to said output transducer.

2. A device for producing delayed decaying electrical signals including in combination, input and output transducers for respectively producing mechanical motion from electrical signals and electrical signals from mechanical motion, a pair of vibration transmitting members, each having an end coupled to a respective one of said transducers, reflective means suspending the ends of said vibration transmitting members opposite said transducers and providing reflection of vibrations therein in differing phase relationships, and means intermediate the ends of said vibration transmitting means and coupling vibrations therebetween to provide a primary path for vibrations from said input transducer to said output transducer, with said reflective means providing a plurality of secondary paths for vibration.

3. A device for producing delayed decaying electrical signals including in combination, input and output transducers for respectively producing mechanical motion from electrical signals and electrical signals from mechanical motion, a pair of vibration transmitting members, each having first and second ends with said first ends thereof coupled to respective ones of said transducers, first reflective means suspending said second end of one of said vibration transmitting members to reflect vibrations in phase, second reflective means suspending said second end of the other of said vibration transmitting members to reflect vibrations out of phase, and means intermediate the ends of said vibration transmitting members and coupling vibrations therebetween to provide a primary path for vibrations from said input transducers to said output transducer, with said first and second reflective means providing a plurality of secondary paths for vibrations.

4. A device for producing delayed decaying electrical signals including in combination, input and output transducers for respectively producing torsional motion from electrical signals and electrical signals from torsional motion, first and second vibration transmitting helical springs, each having a first and second end With said first ends thereof coupled to a respective one of said transducers, said second end of said first spring being freely suspended to reflect vibrations in phase, said second end of said second spring being rigidly suspended to reflect vibrations 180 out of phase, a link intermediate the ends of said springs and coupling vibrations therebetween to provide a primary path for vibrations from said input transducer to said output transducer, with a plurality of secondary paths for vibrations resulting from the reflective suspension of said first and second springs.

5. A device for producing delayed decaying electrical signals including in combination, input and output transducers for respectively producing torsional motion from electrical signals and electrical signals from torsional motion, first and second vibration transmitting helical springs, each having one end coupled to a respective one of said transducers, reflective means suspending the ends of said vibration transmitting springs opposite said transducer with one of said springs being suspended to be a one-quarter wavelength resonator and the other of said springs being suspended to be a half-Wavelength resonator, and means intermediate the ends of said springs and coupling torsional vibrations therebetween to provide a path for vibration from said input transducer to said output transducer, said springs providing a plurality of standing Waves with the rwonant harmonic peaks thereof falling intermediate each other in an even relationship.

6. A device for producing delayed decaying electrical signals including in combination, input and output transducers for respectively producing mechanical motion from electrical signals and electrical signals from mechanical motion, a pair of vibration transmitting helical springs of equal length, each having an end coupled to a respective one of said transducers, first reflective means freely suspending the other end of one of said springs to reflect vibrations in phase, second reflective means rigidly suspending the other end of the other of said springs to reflect vibrations 180 out of phase, a link intermediate the middle and the suspended ends of said helical springs and coup-ling vibrations therebetween to provide a primary path for vibrations from said input transducer to said output transducer, With said reflective means providing a plurality of secondary paths for vibrations whereby said device produces delayed decaying signals at said output transducer.

'7. A device for producing delayed decaying electrical signals including in combination, a plurality of helical springs, means supporting said springs to permit torsional motion thereof, means intermediate the ends of said springs intercoupling the same to transfer torsional motion therebetween, and input and output transducer means coupled to said means for supporting said springs for respectively producing motion from electrical signals and electrical signals from motion in said device.

References Cited by the Examiner HERMAN KARL SAALBACH, Primary Examiner. E. LIEBERMAN, R. F. HUNT, Assistant Examiners. 

7. A DEVICE FOR PRODUCING DELAYED DECAYING ELECTRICAL SIGNALS INCLUDING IN COMBINATION, A PLURALITY OF HELICAL SPRINGS, MEANS SUPPORTING SAID SPRINGS TO PERMIT TORSIONAL MOTION THEREOF, MEANS INTERMEDIATE THE ENDS OF SAID SPRINGS INTERCOUPLING THE SAME TO TRANSFER TORSIONAL MOTION THEREBETWEEN, SAID INPUT AND OUTPUT TRANSDUCER MEANS COUPLED TO SAID MEANS FOR SUPPORTING SAID SPRINGS FOR RESPECTIVELY PRODUCING MOTION FROM ELECTRICAL SIGNALS AND ELECTRICAL SIGNALS FROM MOTION IN SAID DEVICE. 